1. Field of the Invention
The present invention relates generally to cooldown chambers for use in integrated circuit fabrication processes. More particularly, the present invention relates to an improved cooldown chamber providing more rapid cooldown of a substrate containing a lower volume of cooling gas.
2. Background of the Related Art
Fabricating integrated circuit structures on silicon wafers requires the deposition and etching of multiple layers of metals, dielectrics, and semiconductors. These processes include chemical vapor deposition, physical vapor deposition, dielectric depositions, various etching processes, and the like. Many of these processes take place in the presence of a plasma or otherwise occur at elevated temperatures. Therefore, the substrate and film layers formed thereon must periodically be cooled either between the process steps or before withdrawal of the wafer from a controlled environment, such as that of a cluster tool.
Typically, the cooling of a substrate and the films formed thereon is performed in a cooldown chamber. This chamber is attached to a cluster tool and is dedicated to the cooling of wafers to temperatures at which the integrity of the films is preserved. Typical cooldown chambers comprise an enclosure having a slit valve allowing access from a buffer chamber of a cluster tool. The cooldown chamber, like the buffer chamber, is preferably operated at relatively high pressures of about 20 torr. The cooldown chamber is operated at these pressures or greater in order to achieve sufficient thermal heat conduction from the substrate to the surrounding environment. Cooldown chambers are equipped with a pedestal having a cooling member formed therein to receive a substrate and absorb a majority of the thermal energy released from the substrate. In applications where the cooldown chamber is operated at a pressure higher than that of the buffer chamber, it has been common to incorporate a slit valve between the cooldown chamber and the buffer chamber to contain the higher pressure. Once the substrate has been cooled, the slit valve is opened and the higher pressure gas from the cooldown chamber is allowed to escape into the buffer chamber. While this creates a high pressure event in the buffer chamber, this higher pressure has typically been tolerated.
To increase the throughput of substrates through a cluster tool, it is generally beneficial that each individual process step occur as quickly as possible. In a case of a cooldown chamber, it is desirable for the substrate to be cooled in an efficient and timely fashion and sent on to the next chamber for further processing, leaving the cooldown chamber ready to cool yet another substrate. It is therefore desirable to maximize the efficient cooling of the substrate in a cooldown chamber.
The transfer of thermal energy from the substrate to the cooling member can occur, in varying proportions, through thermal radiation, thermal conduction and convection. In order to provide efficient thermal conduction from the substrate to any solid body within the cooldown chamber, the cooldown chamber is typically operated at pressures of at least 20 torr. These pressures are necessary to provide a sufficient density of gas molecules to transfer heat between adjacent surfaces. Even face-to-face contact between the substrate surface and a cooling member does not provide efficient heat transfer in a vacuum, because the uneven surfaces of the substrate and cooling member create pockets which, in the absence of sufficient gas molecules, provide insulation between the two members.
Furthermore, the rate of heat transfer due to thermal radiation and convection is much slower than thermal conduction. Therefore, efficient cooling chamber designs rely primarily on thermal conduction. In operation of the cooldown chamber, a robot blade inserts a hot substrate through the slit valve into the cooldown chamber, where the substrate is received on a plurality of receiving pins. The robot blade is then withdrawn from the chamber and the slit valve is closed. The lift pins may then lower the wafer down onto the substrate receiving surface of the cooling member. With the substrate positioned on the cooling member, the substrate is allowed to cool over a period of time sufficient to stabilize the substrate films. After this period of time has passed, the slit valve is opened and the substrate is withdrawn from the cooldown chamber by the robot and transferred to either a subsequent process or the load lock cassette for withdrawal from the cluster tool.
However, this simple cooldown chamber is incompatible with several processes commonly used in semiconductor fabrication. For example, if the cooldown chamber just described was attached to a cluster tool having a PVD deposition chamber attached to a common buffer chamber, the high pressure excursion caused by the opening of the slit valve between the cooldown chamber and the buffer chamber can easily cause contamination of the PVD chamber upon opening of the PVD slit valve. Therefore, it is critical that the cooldown chamber slit valve and PVD chamber slit valve never be opened at the same time. In fact, the PVD chamber slit valve should not be opened until the pressure in the buffer chamber is again drawn down to its operating pressure. While the cooldown chamber is typically filled within an inert gas, high pressure gas rushing through the buffer chamber into the PVD chamber could contaminate the PVD chamber with particles generated by the robot and other substrate transfer processes.
Pressure excursions in the buffer chamber can be reduced or eliminated by the addition of a vacuum pump to the cooldown chamber. In operation, the vacuum pump would be used to reduce the pressure in the cooldown chamber to that equal to or below the pressure in the buffer chamber prior to opening the slit valve and removing the substrate. However, this solution adds additional auxiliary equipment, maintenance and expense to the cluster tool. Furthermore, and perhaps more importantly, adding a vacuum pumping step to the substrate cooling process adds a considerable amount of processing time.
Therefore, there is a need for a cooldown chamber that provides rapid cooling of a substrate. More particularly, there is a need for a cooldown chamber that is compatible with low pressure processes. It would be desirable if the cooldown chamber could operate at high pressure to maximize thermal conductance, yet minimize or eliminate pressure excursions in the buffer chamber. It would be further desirable if the cooldown chamber required only a minimum volume of high pressure gas in order to reduce the magnitude of the pressure excursion or, alternatively, reduce the amount of time required to pump and evacuate the gas.
The present invention provides a cooldown chamber having an enclosure and a passageway between the enclosure and a buffer chamber of a cluster tool. The cooldown chamber utilizes a first cooling member coupled to an inside wall of the enclosure and a second cooling member coupled to a pedestal for receiving a substrate thereon, wherein the second cooling member can be selectively positioned adjacent the first cooling member to form a cooling region therebetween. A gas source provides an inert gas, preferably nitrogen or helium, to the cooling region. It is preferred that the cooling members be in fluid communication with a source of a cooling fluid, such as chilled water.
In one aspect of the invention, the passageway between the enclosure and a buffer chamber contains a slit valve to isolate the cooldown chamber. The cooldown chamber may also include an exhaust port and a vacuum pump for evacuating the inert gas from the cooldown chamber before opening the slit valve.
In another aspect of the invention, the cooldown chamber has a second passageway between the enclosure and a second buffer chamber of a cluster tool, wherein the second passageway includes a slit valve. In this manner, the cooldown chamber can also serve as a loadlock between buffer chambers that also operate at different conditions or pressures.
In yet another aspect of the invention, a substrate is cooled at an increased rate by positioning the cooling members within about 0.01 and about 0.03 inches from the substrate surfaces. Furthermore, one or both of the cooling members may have a rim extending a sufficient distance towards the other cooling member that the cooling members can be selectively positioned to form a cooling cavity containing the substrate. The cooling rate is also increased by providing an inert gas to the cavity at a pressure greater than about 5 torr in the cavity, preferably between about 5 and about 30 torr, and most preferably about 20 torr.
The invention also provides a method for operating a high pressure cooldown chamber on a cluster tool having a low pressure transfer chamber, comprising the steps of (a) transferring a substrate from the transfer chamber into the cooldown chamber, wherein the cooldown chamber comprises: (i) an enclosure; (ii) a passageway between the enclosure and a buffer chamber of a cluster tool; (iii) a first cooling member coupled to an inside wall of the enclosure; (iv) a second cooling member coupled to a pedestal for receiving a substrate thereon; and (v) a gas source for providing inert gas between the first and second cooling members; (b) receiving the substrate on the second cooling member; (c) positioning the second cooling member in a sealing relationship with the first cooling member to form a cooling cavity therebetween, wherein the cavity contains the substrate; (d) flowing an inert gas into the cooling cavity at a pressure that is greater than the pressure in the transfer chamber; and (e) cooling the substrate. It is also preferred that the gas-containing volume of the cooling cavity be less than the cooldown chamber volume, preferably less than about 10 percent.